1. Field of the Invention
The invention relates to an inductive proximity switch with an at least partially metal, preferably high-grade steel, especially preferably VA steel housing, especially with a housing which is formed at least partially of a nonmagnetic steel, with at least one transmitting coil, with two receiving coils which are connected in series in opposite directions and which are located symmetrically to the transmitting coil, and with an evaluation circuit which is connected to the receiving coils.
2. Description of Related Art
Inductive proximity switches, therefore electronic switching devices, are made without contacts and have been used for almost forty years largely in place of electrical, mechanically activated switching devices which are made with contacts, especially in electrical and electronic switching, measurement, and control circuits.
With inductive proximity switches it is indicated whether an electrically conductive, generally a metallic influence element, hereinafter always called a target, has approached the proximity switch far enough. If the target has approached the proximity switch far enough, an electronic switch which belongs to the inductive proximity switch is reversed; When the proximity switch is made as a make contact, the previously nonconductive electronic switch now becomes conductive, while in a proximity switch made as a break contact, the previously conductive electronic switch now blocks.
There are currently inductive proximity switches of various types. In the first type of inductive proximity switches, they include an oscillator. Then, it applies that part of the oscillator is a receiving coil or the oscillator with its “input” is connected to a receiving coil and that the oscillator is part of the evaluation circuit or the evaluation circuit is connected to the output of the oscillator. In inductive proximity switches of the first type, which include an oscillator, it applies to the oscillator, as long as the target has not yet reached a given distance to the inductive proximity switch, K×V=1 with K=feedback factor and V=magnification factor of the oscillator; i.e., the oscillator oscillates. When the target reaches a given distance, this generally leads to a reduction of the feedback factor K and magnification factor V so that K×V<1; i.e., the oscillations of the oscillator decay or the oscillator ceases to oscillate. Regardless of the state of the oscillator or the amplitude of the output voltage of the oscillator, an electronic circuit belonging to the evaluation circuit is controlled.
For the described inductive proximity switches of the first type, to detect the approach of a target, the so-called eddy current process is used in which the eddy current losses are evaluated which form when a target is moved into an alternating electromagnetic field which proceeds from the inductive proximity switch.
The eddy current process has the major disadvantage that the operating distance of the inductive proximity switch is dependent on the material of the target; if reference is made to the operating distance of an inductive proximity switch for a ferromagnetic target, the operating distance of the same inductive proximity switch for a non-ferromagnetic target is, for example, only roughly 50%. Relative to the operating distance which a certain inductive proximity switch has for a ferromagnetic target, therefore, a so-called correction factor must be used for a non-ferromagnetic target.
To have to use a correction factor in inductive proximity switches depending on the material of the target has been recognized to be a disadvantage for many years. Consequently, the technical field has already extensively addressed the problem of making an inductive proximity switch such that it has a correction factor of 1, i.e., so that a correction is not necessary (see German patent disclosure documents and patents 32 25 193 (U.S. Pat. No. 4,553,040); 37 14 433 (U.S. Pat. No. 4,879,531); 38 14 131; 38 40 532; 39 12 946 (U.S. Pat. No. 5,012,206); 39 16 916; 40 21 164; 40 31 252 (U.S. Pat. No. 5,264,733); 43 30 140; and 197 40 774.
In the second type of inductive proximity switches, an oscillator is not absolutely essential. In these inductive proximity switches, the influencing of a receiving coil which can be achieved by the target is evaluated differently by the evaluation circuit connected to the receiving coil. In this case, an alternating current is fed into the transmitting coil. Part of the resulting alternating electromagnetic field penetrates the receiving coil and induces in it a voltage which is dependent on the influence distance of the target. In the simplest case, a threshold switch is connected to the receiving coil as the input-side part of the evaluation circuit and responds to whether the voltage on the receiving coil is above or below a given threshold value; the voltage on the receiving coil is called the indicator voltage because the receiving coil is the actual indicator for whether the inductive proximity switch is significantly influenced by the target or not. Instead of a simple threshold switch, the evaluation circuit on the input side can also have an amplifier, a demodulator, a threshold switch and an additional switching amplifier.
Therefore, in inductive proximity switches of the latter described type, to detect the approach of a target, the above described eddy current process is not used. Instead, the so-called transformer process is used in which the target influences the magnetic coupling between the transmitting coil and the receiving coil, and thus, the magnitude of the voltage induced in the receiving coil.
In the inductive proximity switch which was initially described specifically and which, among others, is known from German patent disclosure documents 198 34 071 (U.S. Pat. No. 6,545,464) and 100 12 830 (U.S. Pat. No. 6,657,323), which therefore, in addition to the transmitting coil, has two receiving coils which are connected in series in opposite directions and which are located symmetrically relative to the transmitting coil. The transformer method is used in a special configuration, hereinafter called the transformer difference method. In this connection, in the two receiving coils, voltages are induced which have opposite polarity. The series connection of the two receiving coils then leads to the resulting voltage on the series connection of the two receiving coils being zero when the voltages induced in the two receiving coils are exactly the same in terms of amount and are exactly in opposing phase.
Inductive proximity switches of the type which underlie the invention and which are to be made and developed in accordance with the invention are now built such that, in the uninfluenced state, the resulting voltage on the series connection of the two receiving coils is not zero, but is very small, for example, 5 mV. For the uninfluenced state of the inductive proximity switch under consideration, a resulting voltage which differs from zero on the series connection of the two receiving coils is chosen because the deviation of the resulting voltage which occurs when the inductive proximity switch is influenced can be better detected and processed by the evaluation circuit when the initial value for the uninfluenced proximity switch is not equal to zero.
When a target approaches an inductive proximity switch of the above described type, in this way, the magnetic coupling between the transmitting coil, on the one hand, and the receiving coils, on the other hand, is asymmetrically influenced. This results in the fact that, in the two receiving coils, voltages are induced which are no longer oppositely equal, so that, as a result, on the series connection of the two receiving coils, a voltage forms which deviates from the voltage which forms when the proximity switch is not influenced. If this voltage exceeds a given threshold value, the signal is evaluated as a “proximity switch influenced” signal.
In addition to the described problem of the necessity of a correction factor, for inductive proximity switches there are other criteria which can be important. For example, inductive proximity switches should often have a relatively large operating distance at a given overall size. The operating distance for which the inductive proximity switches are designed should be largely stable, especially should be independent of temperature as much as possible.
In various applications, for example, in the foodstuffs industry, so-called all-metal switches are required, i.e., inductive proximity switches which have a metal, preferably a high-grade steel housing, because permeation is or cannot be reliably enough prevented in a plastic housing.